Subject: Comments on the weak lensing report
From: strauss@astro.Princeton.EDU
Submitted: Thu, 5 Dec 2002 09:18:23 -0500 (EST)
Message number: 26
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Hi Tony,
Thanks very much for your detailed memo on weak lensing. As you
write in your first paragraph, the list of questions that you tackle
here are things that each of the work groups in the SWG need to put
thought to.
Did you get the input of the rest of the weak lensing work group in
preparing this? If not, I would urge the others signed up to work on
weak lensing within the SWG to put in their two cents; are they
basically happy with Tony's general approach, assumptions, plan for
further work, etc?
Let me ask a few questions of my own, which probably mostly reflect
my ignorance of the field. Reading sequentially through the document:
You have assumed Gaussian seeing in the simulations that you've
done. As you know, real seeing is more "interesting" than that; the
SDSS, for example, has modelled it as the sum of two Gaussians, plus a
power-law tail. It will be interesting to see what effect this has on
shape measurements of galaxies. Any thoughts on this?
I'm not sure I understand quite all the items listed in your list of
requirements to be considered. But a few comments. In particular, as
you mention later in the document, we'll be doing the shape
measurements off the stacked images, perhaps several hundred for a
given spot on the sky. Anisotropies and spatial variations of the PSF
should be averaged over to a certain extent in this stacking; this
will allow us presumably to relax any criteria on the uniformity and
isotropy of the PSF in a single exposure.
You have an item (#6) having to do with astrometry. Can you explain
that further? How do astrometric errors feed into errors on shear?
You mention that existing 4-meter telescopes show substantial PSF
ellipticity (several percent), and say that this reflects the fact
that these telescopes were not designed for such precision work. What
in fact is the leading cause of this ellipticity? Improper mirror
support? Jitter in the guiding? Poor tracking? All of the above? I
agree in principle that if we put our minds to it, we can minimize
these effects in the LSST, but we need to know exactly what we're
fighting against.
*********Science Drivers*****************
This may all exist in the literature (and some references would be
useful), but let me see if I understand the jist of your discussion
here. There are two separate weak-lensing studies that can be done
with LSST-like data: the power spectrum of the cosmic shear, and
direct counts of (presumably virialized) overdensities. The former is
presumably done on larger scales than the latter. The former is
directly predictable from linear theory (if one knows one's selection
function), while the latter can be predicted using a combination of
some version of Press-Schechter, and N-body simulations. Is this
correct? It seems that most of the discussion here (and also in Joe
Hennawi's work) has been focussed on the latter; is it the more
powerful, or simply better-understood of the two?
In this context then, you make reference to error ellipsoids in the
Omega_L/Omega_M plane (or should it be the w/Omega_M plane? The full
relevant parameter space is not completely clear to me, what with the
MAP data about to arrive...). Exactly which of the above analyses
gives rise to these constraints is unclear.
In this context, there is reference to the relevant depths that are
sensitive to various effects, especially w. It is stated that the
dark energy manifests itself most strongly for z<0.8; is this simply a
statement that at higher redshifts, Omega_M approaches 1, and all
models become degenerate?
In this context, there is discussion of a lensing survey to 26th
magnitude. What exactly does this magnitude limit mean? One of
course can't carry out detailed shape measurements for objects with
5-sigma detections, so I am guessing that this number refers to a
faintest galaxy at which the shape can be measured. Is this right?
Can one translate this into something like a 5-sigma limit for point
sources (which can be related to exposure time given the seeing, the
telescope aperture, and sky brightness).
***************Weak lensing in 10 years*************************
The estimate of where the lensing field will be in ten years doesn't
mention VISTA. I do know that they plan to start with a near-IR
survey, but isn't it likely that on the 10-year timescale, they will
expand to wide-field optical imaging as well?
*************LSST's contribution************************
You mention that we will want to go to 26th mag in 5 bands for
photometric redshifts, and then deeper in one band. Does one gain
anything for shape measurements if one co-adds the data in different
bands? These are independent photons, and at least for early-type
galaxies, substructure and color gradients are unlikely to cause much
trouble.
You say that for the optical shape measurements, you want just to
co-add the best-seeing images. I've heard Nick Kaiser quoted
second-hand that the optimal thing to do is to convolve each image by
its effective seeing, then co-add, then deconvolve with the coaddition
of the seeing kernels. This then doesn't involve throwing away any
data, but I've never seen a demonstration of its efficacy.
You say that the spectrum of over-densities at very large angular
scales is a useful diagnostic. I lost you; are you speaking of the
power spectrum of fluctuations (see my questions above)? Or are you
speaking of virialized structures? I lost you here. In the
following paragraph, you talk about measurement of cosmic variance;
aren't you simply talking about the measurement of P(k) of the shear
signal on the largest scales? You do make reference to a non-Gaussian
signature, but I am not sure I understand what you are referring to
here.
When you talk about strong lensing, you say that there is only one
system in which multiple arcs from a single background galaxy is
known; I assume it is the one you've worked on with Ed Turner and Wes
Colley (I don't remember its telephone number). But there are *lots*
of clusters now with substantial arc systems (Abell 370 and 2218 being
among the classics), even if none is as clean as the one you had in
mind. Aren't those also useful for this purpose?
*********************System requirements********************
I remain unclear on some of the technicalities of what you write
here. I don't know what 'unmodeled detector-focal place errors' are,
which you say are a major effect in PSF shear systematics. I also did
not understand the statement that "the main benefit of a stack of 200
images will come from getting better source shear measurements." I
lost you; what is a 'source shear measurement'?
You then say that the LSST should have PSF ellipticity a factor of
10 below that of the 4m. See my question above; what exactly is
driving the ellipticity on the 4m, and how can we be confident that we
can get rid of these systematics on the LSST?
Those are my questions. This document is a wonderful start to a
full science case and formal requirements for the weak lensing. One
thing that I think we need more of here is some of the cosmological
formalism (i.e., equations!) that lie behind the various statements
here. Again, I suspect most of it is in the literature, but we should
summarize it in a future version of this document.
I am eager to hear comments from the rest of the SWG, both on the
weak lensing specifics, and whether the other work groups on other
subjects are in a position to put out similarly detailed reports.
Many thanks,
Michael Strauss
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